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The Intestinal Protozoa

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The Intestinal Protozoa A. Introduction 1. The Phylum Protozoa is classified into four major subdivisions according to the methods of locomotion and reproduction. a. The amoebae (Superclass Sarcodina, Class Rhizopodea move by means of pseudopodia and reproduce exclusively by asexual binary division. b. The flagellates (Superclass Mastigophora, Class Zoomasitgophorea) typically move by long, whiplike flagella and reproduce by binary fission. c. The ciliates (Subphylum Ciliophora, Class Ciliata) are propelled by rows of cilia that beat with a synchronized wavelike motion. d. The sporozoans (Subphylum Sporozoa) lack specialized organelles of motility but have a unique type of life cycle, alternating between sexual and asexual reproductive cycles (alternation of generations). e. Number of species - there are about 45,000 protozoan species; around 8000 are parasitic, and around 25 species are important to humans. 2. Diagnosis - must learn to differentiate between the harmless and the medically important. This is most often based upon the morphology of respective organisms. 3. Transmission - mostly person-to-person, via fecal-oral route; fecally contaminated food or water important (organisms remain viable for around 30 days in cool moist environment with few bacteria; other means of transmission include sexual, insects, animals (zoonoses). B. Structures 1. trophozoite - the motile vegetative stage; multiplies via binary fission; colonizes host. 2. cyst - the inactive, non-motile, infective stage; survives the environment due to the presence of a cyst wall. 3. nuclear structure - important in the identification of organisms and species differentiation. 4. diagnostic features a. size - helpful in identifying organisms; must have calibrated objectives on the microscope in order to measure accurately. b. type of motility - directional or non-directional; sluggish or fast. c. cytoplasmic inclusions - chromatoid bars (coalesced RNA); red blood cells; food vacuoles containing bacteria, yeast, etc. d. appearance of cytoplasm - smooth & clean or vacuolated. 5. endosome - also called the “karyosome,” this is a mass of chromatin within IntProt.doc 1 of 15 nucleus. The size, shape, and location of this structure are helpful in identification of organisms. 6. chromatin - nuclear DNA 7. chromatoid body or “bar” - coalesced RNA within the cytoplasm in the cyst stage. This is not always present, but when it is, its size and shape are helpful in determining species identification. II. Class Lobosea - The Amoebae A. Life cycle - 1. The definitive host ingests the infective cyst stage from fecal contamination in environment. 2. The cyst passes into the small intestine & excystation occurs with transformation to the trophozoite stage. 3. Trophozoites in the large intestine colonize the host by multiplying asexually via binary fission. They can remain in the lumen or invade the wall of the intestine (pathogenic species only) & multiply, from here they can be transported via the circulation to other organs (liver, lungs, etc.). 4. Cysts and trophozoites are passed in the feces of the infected host. 5. Infective stage - the mature cyst. 6. Diagnostic stage - the trophozoite or cyst in stool or tissue specimens. B. Genus Entamoeba - contains the most important of the amoebae causing disease in humans. 1. Entamoeba histolytica Entamoeba histolytica trophozoites Entamoeba histolytica trophozoite with ingested red blood cells with ingested red blood cells Entamoeba histolytica cyst Entamoeba histolytica cyst E. histolytica cyst with chromatoid bar with chromatoid bar (iodine stain) a. Epidemiology - Occurs worldwide; the highest incidence and prevalence is found in areas with poor sanitation where as many as 80% of a population may be infected. Highest in children >5 years of age; more IntProt.doc 2 of 15 prevalent in males than in females; common in mental hospitals, prisons, orphanages. b. Pathology and Clinical Manifestations - the most pathogenic of all; causes amoebic dysentery; can become extra-intestinal (liver, lungs, etc.; can be fatal. c. Morphology & Laboratory Identification - the trophozoite ranges between 12 and 30 microns in diameter. Its nucleus has an even distribution of peripheral chromatin and a small, compact, centrally located karyosome. The cytoplasm is typically smooth and granular with inclusions, if present, containing red blood cells. The cyst ranges between 10 and 30 microns in diameter and contains four nuclei when mature. Cigar-shaped chromatoid bars may be present in some cysts. d. Treatment - depends upon location of infection (lumen vs. tissue) e. Distribution - worldwide, mostly in tropics and sub-tropics. f. Note - chronic infections may last for years; they are often confused with other conditions (colitis, cancer). Hepatic abscess is the most common and dangerous complication. 2. Entamoeba hartmanni Entamoeba hartmanni trophozoite Entamoeba hartmanni trophozoite trichrome stain iron-hematoxylin stain Entamoeba hartmanni cyst (iodine stain) a. Formerly called the “small race” of Entamoeba histolytica. b. Technologists must be able to differentiate this organism from E. histolytica because E. hartmanni is non-pathogenic. c. Morphology & Laboratory Identification - this organism is very similar to E. histolytica. The difference lies in the respective sizes of the organisms. Trophozoites of Entamoeba hartmanni will measure less than 12 microns, while cysts will measure less than 10 microns. It is strongly recommended that more than one organism be measured when determining size. IntProt.doc 3 of 15 2. Entamoeba coli Entamoeba coli trophozoite E. coli trophozoite E. coli cyst E. coli cyst (iodine) a. Morphology - trophozoites range from 10 to 35 microns in diameter; cysts range from 10 to 30 microns in diameter and contain 8 to 16 nuclei when mature; the nucleus exhibits an eccentric karyosome with irregular, coarse chromatin. The cytoplasm is heavily vacuolated, containing yeast, bacteria, and debris. b. Significance - this is a harmless commensal but must be differentiated from the pathogens. 2. Entamoeba gingivalis Entamoeba gingivalis trophozoite a. Infective site - in the mouth; the organism thrives in diseased gums, but is not considered a causal agent or pathogen. If swallowed, it is destroyed in stomach. b. Transmission - contact with fomites (drinking glasses, eating utensils, etc.; kissing. c. Morphology - resembles E. histolytica, but has no cyst stage. It is the only species, which ingests leucocytes. C. Other amoebae of significance 1. Endolimax nana Endolimax nana trophozoite E. nana cyst E. nana cyst (iodine) a. Occurrence - occurs in about 14% of the US population; 21% worldwide. IntProt.doc 4 of 15 b. Pathogenicity - none. c. Morphology - trophozoites range from 5 to 10 microns in diameter. The nucleus contains a large, blot-like karyosome and has little or no peripheral chromatin. Cysts are usually sub-oval, measuring 4 to 6 by 6 to 10 microns 2. Iodamoeba butschlii Iodamoeba butschlii trophozoite I. Butschlii cyst I. Butschlii cyst (iodine) a. Pathogenicity - none. b. Morphology – often called iodine cyst - striking characteristic is large glycogen vacuole - stains dark brown with iodine. 3. Naegleria fowleri Naegleria fowleri trophozoite in CSF N. fowleri trophozoites a. Classification - an ameboflagellate and a free-living organism; it alternates between amoeboid and bi-flagellated forms; only the amoeboid form is found in tissues. Disease occurs during summer months. Occurs only in fresh water, can tolerate chlorinated swimming pools. b. Life cycle - the amoeba gains entry via the nasal mucosa, usually during a swimming event; it moves along the olfactory nerve, gaining access to the brain via the cribriform plate. It rapidly colonizes the brain, and cases are invariably fatal. Infections do not spread form person-to-person. c. Symptoms - Symptoms are dramatic and rapidly progressive. Headache, fever, nausea & vomiting occur within 1 to 2 days. Meningoencephalitis, irrational behavior, coma & death usually occur within 9 days of exposure. d. Diagnosis - Usually made at autopsy. CSF contains a large number of segmenters with no bacteria present. Motile amoebae can be seen in wet mounts of CSF. Early diagnosis is critical. CSF specimens can be plated onto non-nutrient agar containing a lawn of bacteria; amoebae, which grow out, can be transferred into sterile filtered water to determine IntProt.doc 5 of 15 if the organisms present transform into the bi-flagellated stage. e. Treatment - Amphotericin B (Intravenous and Intrathecal) with rifampin is the only regimen to date that has proven successful in saving a patient. 4. Acanthamoeba spp. Acanthamoeba spp. trophozoite a. Life cycle - also a free-living amoeba. The amoeba reaches the brain hematogenously after entering a wound or lesion on the skin. More commonly, the organism is associated with getting into eyes via contaminated or homemade cleaning solutions. b. Symptoms - slow onset (10 or more days). Presents as chronic, granulomatous lesions in brain. In eye lesions, the infection resembles a herpes virus infection. c. Acanthamoeba keratitis - affects healthy persons; sometimes associated with eye trauma; most recently has been associated with users of extended-wear contact lenses. Cysts are resistant to drying and disinfection via chlorine. III. Superclass Mastigophora - the flagellates; members of this group can inhabit mouth, bloodstream, tissues, gastrointestinal, or urogenital tracts. A. Members: Giardia lamblia Dientamoeba fragilis Chilomastix
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